Maziarz M., Wesołowski T. 2010. Timing of breeding and nestling diet of Wood Warbler Phylloscopus sibilatrix in relation to changing food supply. Bird Study 57: 540-552.

Capsule Wood Warblers did not match their reproduction to the caterpillar peak
Aims To study the timing of Wood Warblers’ breeding and nestling diet in relation to caterpillar abundance under primeval conditions in the Białowieża National Park, Poland.
Methods Observations of food brought by parents. Inter-year and seasonal changes in availability of folivorous caterpillars were assessed by direct counts and caterpillar frass collection.
Results Maximum food requirements occurred two weeks after the peak of caterpillar abundance. The mismatch had no effect on nestlings’ development. Diet varied little across years and habitats, but varied strongly within a season. Following the decline of ‘green’ caterpillars, their proportion in the Wood Warbler diet strongly declined. Caterpillars were replaced by winged insects. Small nestlings received more spiders than older ones.
Conclusion Timing of Wood Warbler breeding in Białowieża National Park was constrained by the females’ arrival time. Birds did not match their reproduction to the caterpillar peak. ‘Green’ caterpillars were the preferred food for nestlings; birds responded to the caterpillar decrease by feeding smaller proportions of them and switching to alternative prey, namely winged insects.